I have trained in TKD for 5+ years, hell i was training for the olympics at one point, and now i've been training straight muay thai for about 2years.
The spinning back kick may very well be the most powerful kick the human body can generate...
but as far as consistently damaging your opponent it cant compare to the thai roundhouse, well not just thai.... the roundhouse in general to be honest. That is, in terms of probability of landing the technique and how often you can land it based on how many times you throw it.
A spinning back kick to the solar plexus could easily end a fight, but do you know how hard it would be to land a solid few in a live kickboxing match?
A roundhouse may not be as powerful, but a decent powered roundkick to the liver will put your opponent down writhing in pain, he will even be sick for a few days afterwards.
Muay thai is by no means the ultimate standup style, but it is very complete, it is almost a "pick up and play" art. In my opinion, it lacks footwork and fluidity of punch kick combinations, something Savate and American kickboxing are well known for. It also sometimes lacks sophisticated punch defense and tactics. What alot of people call "hands".
In muay thai, every strike(kick) that is thrown, is to be thrown will 100% power, unless it is a feint. This is muay thai's strength, as well as it's weakness.
This really got me to thinking, and going outside my own styles to learn about other arts and integrate it into my style. I will always be open minded no matter what, because without that i wont be able to grow as a martial artist. I took a page out of Bruce lee's book.
Don't know why i started going off like that, but yeah anyway, as far as raw power goes, its probably the spinning back kick.